1. Field of the Invention
The present invention relates to processes for the separation of acid from sugars obtained from the acid hydrolysis of cellulosic and/or hemicellulosic material. The present invention further relates to a method for removing the heavy metals associated with the acid/sugar solutions that are obtained from municipal solid waste (MSW) or sewage sludge which may contain cellulosic and/or hemicellulosic materials.
2. Related Art
U.S. Pat. No. 5,580,389 discloses methods for separating acids and sugars resulting from strong acid hydrolysis of biomass containing cellulose and hemicellulose. The cellulose and hemicellulose is first decrystallized and then hydrolyzed to produce a hydrolysate containing both sugars and acid. Silica present in the biomass can then be removed for further processing. The remaining solids are then subjected to a second decrystallization and hydrolyzation to optimize the sugar yields. A method for separating the sugars from the acid in the hydrolysate is also disclosed. The resultant sugar stream containing both hexoses and pentoses can then be fermented. In particular, the method of separating the sugars from the hydrolysate involves the use of a resin separation unit wherein the sugars are adsorbed on a strong acid resin. Liquids are then added to the resin and an acid stream containing less than 2% sugar is obtained.
U.S. Pat. Nos. 5,407,580 and 5,538,637 disclose a process for separating an ionic component such as acid from a nonionic component such as sugar in polar solutions using ion exclusion technology. In particular, the methods involve control of dispersion, caused by shrinkage of resin by acid within ion exchange columns. The continuous method employs an acid resin which is contacted with an acid-sugar solution whereby a first fraction comprises an enriched fraction of acid and later fractions comprise an enriched fraction of sugar.
U.S. Pat. Nos. 5,407,817 and 5,571,703 disclose methods of processing municipal solid waste to recover materials that may be recycled and to produce ethanol from the cellulosic component. One particular problem of municipal solid waste is the heavy metal component which may inhibit the fermentation of sugars by yeast or other fermentative microorganism. According to these patents, the heavy metals can be removed in any one of two different methods. In the "front end" process, the heavy metals are removed by treating the shredded cellulosic component with dilute sulfuric acid at about 40 to 100.degree. C. to solubilize substantially the heavy metals. The insoluble component of the reaction mixture comprising the cellulose may then be recovered and processed to give ethanol. In the "back end" process, the shredded cellulosic component is directly hydrolyzed with concentrated sulfuric acid. After dilution with water and cooking at elevated temperatures, the insoluble lignin is removed. It was discovered that heavy metals were associated with the insoluble lignin which allows for the removal of the heavy metals from the sugar containing solution.
U.S. Pat. No. 5,651,895 discloses methods of removing heavy metals from certain waste waters by adding an iron salt, and then a base to increase the pH to a level of about 8 to 10. As the base is added, a precipitate containing the heavy metals is formed which is then removed by standard filtration techniques. The process can be carried out continuously or batch wise.